1. Field of the Invention
This invention relates to a method for eluting a metal adsorbed on a chelating agent, more particularly a method for eluting a metal from a chelating agent having adsorbed the metal by using as an eluent an aqueous base solution containing a sulfide.
2. Description of the Prior Art
Elution of a metal adsorbed on a chelating agent has heretofore been conducted generally by bringing a chelating agent having adsorbed the metal into contact with an aqueous solution of a mineral acid such as sulfuric acid, hydrochloric acid or the like. However, in the case of employing such a method, when the chelating agent has a functional group having no acid resistance such as an oxime group, a phosphoric ester group or the like, it is deteriorated in capability by decomposition of the functional group. Therefore, the employment thereof is not desirable.
For eluting a metal such as mercury, cadmium, copper, zinc, lead, tin, gold, silver or the like, for example, from a chelating agent having a special functional group of sulfur series possessing a high bonding strength to heavy metals such as --SH, &gt;C.dbd.S or the like, there is also known an elution method using an aqueous sulfide solution (Japanese Patent Publication Nos 28,274/76 and 35,399/76).
However, in general, employment of an aqueous sulfide solution as an eluent is diadvantageous, for example, in that elution requires a very long time; that since metal sulfides are generally low in solubility, eluted metal is deposited as a sulfide, so that for recovering the metal, a troublesome step of dissolving and recovering the metal again is needed; and that the eluted metal is precipitated as a metal sulfide which is slightly soluble in water, therefore when a solid adsorbent is used, the metal sulfide is accumulated in the adsorbent, so that the pores are clogged therewith, resulting in deterioration of the adsorbent in capability.
In consideration of these circumstances, in order to develop a method for eluting a metal adsorbed on a chelating agent which is free from the disadvantages described above, the present inventors have conducted research and have consequently found that by using as an eluent an aqueous base solution containing a specified amount of a sulfide, the metal can be recovered in a liquid state to a high concentration range with a high elution rate without essentially causing deterioration of the adsorbent, whereby this invention has been accomplished.